1. Field of the Invention
The present invention related to a horizontal synchronization detection technique.
2. Description of Related Art
There are many standards for video signal formats, and video signals having such various formats are inputted to a video display device. A video display device needs to carry out horizontal synchronization detection to reproduce images from input video signals, and various techniques have been proposed for that purpose.
Japanese unexamined patent application publication No. 11-239281 (Patent document 1) discloses a horizontal synchronization detection method in which a pulse period and a pulse width are detected from a synchronization signal component, and a detection signal is outputted on condition that the detected pulse period and detected pulse width are equal to the period and width, respectively, of the horizontal synchronization signal. This method is effective when the pulse period and pulse width of the horizontal synchronization signal are known in advance.
Japanese unexamined patent application publication No. 61-70861 (Patent document 2) discloses another horizontal synchronization detection method in which a pulse period having the same pulse width as the prescribed pulse width (the pulse width of the horizontal synchronization signal) is detected from a synchronization signal component, and the mean value of the detected pulse periods is determined. Then, a detection signal is outputted on condition that the difference between the detected pulse period and the mean value is smaller than a predetermined value. This method can eliminate noises by restricting the width of pulses that are detected as the synchronization signal, when the pulse width of the horizontal synchronization signal of the input video signal is known in advance.
Japanese unexamined patent application publication No. 2004-215004 (Patent document 3) discloses a method in which the pulse width of synchronization signal component that is separated from the video signal is measured, and pulses whose pulse width is within a predetermined reference range are outputted as the horizontal synchronization signal. Furthermore, the frequency of the synchronization signal component is measured, and the type of the video signal is determined based on the measured frequency, and the reference range of pulse width is changed based on the type of the video signal. This method is fundamentally similar to the method of the Patent document 2. However, since the method determines the type of a video signal and changes the reference range of pulse width based on the type, it enables the method of the Patent document 2 to be applied, even if the pulse width of the horizontal synchronization signal is unknown in advance.
Japanese unexamined patent application publication No. 2002-300424 (Patent document 4) discloses a method in which a noise component is masked by generating a mask signal having a frequency corresponding to the frequency of the horizontal synchronization signal to carry out horizontal synchronization detection, and the phase of the mask signal is adjusted when the synchronization signal pulses to be detected are not detected. While this method can mask the noise component, it can also prevent detection failure of horizontal synchronization signal pulses caused by the out-of-phase mask signal.
Japanese patent No. 3028525 (Patent document 5) discloses a method in which a pulse period is measured in a synchronization signal component, and the mask area of the synchronization signal component is adjusted based on the measurement. This method can detect a horizontal synchronization signal even when the pulse width of the horizontal synchronization signal is unknown in advance, and prevent equivalent pulses from being detected as the synchronization signal pulses.
In recent years, the types of video signals have increased, and there have been various pulse periods and pulse widths of the horizontal synchronization signals of video signals. Therefore, although the pulse period and pulse width of an input video signal are often unknown in advance, accurate horizontal synchronization detection needs to be carried out even in such cases.
The method of the Patent document 1 cannot be used unless the pulse period and pulse width of the horizontal synchronization signal of in input video signal are known in advance.
The method of the Patent document 2 detects only pulses having the same width as the pulse width of the horizontal synchronization signal, and uses the mean period of the detected pulses as the reference value to determine whether or not the detected pulses are genuine synchronization pulses. When the horizontal synchronization signal has an overshoot or undershoot, a pulse width cannot be always detected accurately. Therefore, it is difficult to eliminate noises by restricting the width of the detected pulses. Furthermore, when the difference between the widths of noises and the pulse width of the horizontal synchronization signal is small, it is impossible to eliminate noises by restricting the pulse width. When noises are not eliminated by the pulse width, the mean period that is used as the criterion is affected by the noises, and therefore it poses a problem that correct detection results cannot be obtained.
These facts hold true for the Patent document 3. Furthermore, in the case of the Patent document 3, the type of a video signal is determined, and the reference range is changed based on the type. However, since it is difficult to determine the type of a video signal in the state where the signal contains noises, it may misjudge the type of the video signal and change the reference range of pulse width to a wrong range.
The method of the Patent document 4 requires that the frequency of the horizontal synchronization signal of an input video signal is known in advance.
The method of the Patent document 5 measures a pulse period in a synchronization signal component, and adjusts the mask area of the synchronization signal component based on the measurement. However, since it does not carry out a noise elimination process when periods are measured, the measurement result may be affected by noises and the horizontal synchronization detection may not be carried out correctly.